Orthopaedic implants, such as femoral implants for hip joint replacements, are implanted using two main approaches. In one, a cavity within the femur is shaped to receive the shaft of the implant, and following implantation one relies on natural bone regrowth to fill any voids remaining between the implant and the bone. In the other, poly(methylmethacrylate) (PMMA) bone cement is used to coat an internal wall of a similar cavity, and fills any gaps between the implant and the bone when the implant is inserted.
In the first approach, the ingrown bone bonds strongly to the metal of the implant (usually titanium or a specialist steel). In the second, it has been found that the PMMA cement bonds strongly to the bone, but not to the metal implant. However, this is not a drawback, as the healed bone surrounding the implant holds it in place under compression, and at worst the implant may shift in position millimetrically, to seat further into the cavity or to reach a less stressed position for example.
Prostheses such as femoral implants are durable, but can fail, and the lifetime of the recipient is nowadays very often longer than that of the implant. The technique of revision arthoplasty is hence used to revise the implant—i.e. to remove the failed implant and to replace it with a new one.
At this point, removal of an implant secured by ingrowing bone can be difficult, and it is necessary to chisel or cut down through this bone, around the implant shaft (or a stub left by a fractured implant shaft). This is a lengthy and tiring process for both patient and surgeon, and can result in significant accidental damage to adjacent bone of the walls of the femur.
A range of tools have been proposed to ease this process, including tools activatable with longitudinal-mode ultrasonic vibrations, many devised by the present inventor. However, such tools are still far from perfect, and surgeons would always benefit from using tools that operate more rapidly, more accurately, more controllably and/or with less effort.
Removal of an implant implanted with bone cement is initially easier. Once the head of the femur is opened, for example, the implant is straightforward to draw out, in most cases, since there is little or no adhesion between the metal implant and the bone cement coating the walls of the cavity in the femur. However, it is then necessary to remove all of the existing solid bone cement from the cavity, in order that fresh bone cement paste can be applied, to mould around the new implant as it is inserted.
This is again a tedious process, especially adjacent a distal end of the cavity, where the cement tends to form a solid terminal plug rather than a thin layer around the cavity walls. Various tools have been proposed and used, again including a number of tools vibratable with longitudinal-mode ultrasonic vibrations, devised by the present inventor (International Patent Applications Nos. WO93/03676 and WO96/20657 show examples). These tools are useful because heating of PMMA by energy transfer from an ultrasonically-vibrating tool or probe easily raises the polymer above its depolymerisation temperature of around 120° C., causing it to soften to a consistency allowing it to be scooped or scraped out as a flowable paste, rather than having to be chipped or ground out as a solid. Careful acoustic design can also allow differential energisation of the PMMA while avoiding significant heating of bone, for example should there be accidental transitory contact between bone and an activated tip of the tool or head or probe.
However, there are always benefits to the surgeon from improved tools that operate faster, more accurately, more controllably and/or with less effort.
Additionally, longitudinal-mode ultrasonic vibrations, directed along a probe or tool shaft, can lead to problems, as they characteristically have a distal extensional drilling effect, whether required or not, and they project ultrasound energy a considerable distance longitudinally from a distal tip of the tool/probe. This can lead to tissue damage away from the implant site, and can result in physical bone penetration by said distal tip, even with feedback monitoring to halt such effects by spotting the resonance frequency changes that would occur.
Also, PMMA bone cements and other polymers are nowadays used in other fixation techniques, for example fracture fixation of non-load bearing bones using polymers cured using ultraviolet radiation. This produces a rigid “sausage” of cured polymer extending through a lumen of the fixated bone, across the fracture site. However, once the fracture has healed, removal of the polymer implant is usually indicated, which involves removal of a solid length of polymer. There is hence nowadays a need for equipment that can cope with monolithic bodies of bone cement and other polymers, as well as with thinner layers during metal implant revisions.
Most orthopaedic surgery takes place in relatively cramped geometries. Ideally, even if not “keyhole” surgery, orthopaedic surgery should harm surrounding tissues as little as possible, thus requiring access through as narrow an incision as possible. The geometry of the skeleton and surrounding body tissues may in any case make access to a bone cavity difficult, especially if one is trying to operate along the cavity.
There is hence a need for compact tools that are easily manipulable and can be used in tight spaces. Ultrasonically vibratable tools have a proximal handpiece containing the source of vibrations, generally a stack of piezo-electric ceramic plates mounted to a “horn”, which is a sizeable titanium block with a tapering portion, to which a elongate waveguide is fitted, which transmits the ultrasonic vibrations to an operating head, for example. The dimensions and configuration of the stack and horn significantly affect the resonant frequencies and vibrational amplitudes that can be created in a tool, but unfortunately, many current systems thus inevitably have heavy and bulky stack/horn units which the surgeon must hold and manipulate until the tool can be aligned to act on the desired tissues (or bone or cement, as appropriate). It would hence be desirable to provide more compact and lightweight stack/horn arrangements, or alternatives to such arrangements, to ease their use.
Lastly, the precision required of a surgeon can become unreasonable. For example, when working longitudinally down a cavity, through bone holding an implant to an interior of a femur, a very slight inaccuracy could lead to the tip of the tool veering outwardly and damaging the bone of the femoral wall, or alternatively slanting inwardly and contacting the implant, potentially damaging the tool. It would therefore be beneficial if guidance could be provided for the operation of such elongate tools.
It is hence an object of the present invention to provide ultrasonically-vibratable tools, operative heads/probes/outputs for such tools, ultrasound generators for such tools and/or guidance equipment for such tools that obviate some or all of the drawbacks of known systems, as described above.